Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
  • E21B49/08—Obtaining fluid samples or testing fluids, in boreholes or wells
  • E21B49/081—Obtaining fluid samples or testing fluids, in boreholes or wells with down-hole means for trapping a fluid sample
  • E21B49/082—Wire-line fluid samplers

Definitions

• the present invention relates to the field of sensors for monitoring characteristics of fluids.
• the invention relates to a system and method for monitoring chemical species, chemical properties and the like using diamond-based electrodes. Even more particularly, the invention preferably relates to such sensors used for fluid monitoring in- relation to the development of hydrocarbon and water reservoirs .
• the chemicals present may absorb onto the surface of the graphite electrode.
• Various configurations of diamond material have also been recently proposed as electrodes. See, Soh, Kang, Davidson, Wong, Wisitora-at, Swain and Cliffel, "CVD diamond anisotropic film as electrode for electrochemical sensing", Elselvier Science B.V., 2003; Cvacka, Quaisorova, Park, Show, Muck and Swain, "Boron- Doped Diamond Microelectrodes for Use in Capillary Electrophoresis with Electrochemical Detection” ,
• a sensor for monitoring one or more characteristics associated with a fluid preferably comprises a housing; an insulating layer comprising non-conducting diamond positioned within said housing and having a surface exposed directly or indirectly to the fluid; a plurality of microelectrodes each comprising electrically conducting diamond and having a surface exposed directly or indirectly to the fluid; and an electrical circuit in electrical communication with each of the microelectrodes adapted to convert electrical signals from the microelectrodes into at least one signal associated with a characteristic being monitored.
• the size of the exposed surface of each microelectrode is preferably less than 8000 sq. microns, and even more preferably less than 2000 sq. microns.
• the sensor preferably includes at least seven microelectrodes, and even more preferably at least 19 microelectrodes.
• the microelectrodes are preferably arranged within the insulating layer such that the exposed surfaces of the microelectrodes form a regular pattern, even more preferably a hexagonal pattern.
• the distance between two adjacent microelectrodes is preferably at least five times, and even more preferably ten times, the diameter of a circle having an area equal to the area the exposed surface of each microelectrode.
• the insulating layer and the exposed surface each of the microelectrodes is preferably co-planar with the exposed surface of the insulating layer.
• a gas permeable membrane between a main flow of fluid and the exposed surfaces of the insulating layer and the microelectrodes, wherein the sensor is adapted to sense characteristics associated with gas that is allowed to pass through the membrane.
• the thickness of the microelectrode layer is preferably more than 1 mm.
• the characteristics of the fluid being monitored by the sensor can include chemical properties such as pH, the presence and/or concentration of a chemical species such as hydrogen sulphide, or a property of the fluid such as resistivity.
• the sensor is preferably incorporated into a wellbore sampling tool, a production logging tool, or a measurement-while-drilling subassembly.
• the sensor can also form part of a system to monitor fluids produced from or being pumped into wellbores .
• the present invention is also embodied in a method for monitoring one or more characteristics associated with a fluid.
• diamond refers to carbon with characteristic cubic crystalline structures (or crystal lattices). ' Diamond can be single-, poly- or nano- crystalline.
• Figures la and lb show a diamond-based microelectrode array according to a preferred embodiment of the invention
• Figure lc shows a microelectrode array according to another embodiment of the invention
• Figure Id shows a microelectrode array having a square pattern, according to another embodiment of the invention
• Figure 2 show an arrangement of microelectrodes in a diamond layer according another embodiment of the invention
• Figure 3 shows an array of microelectrodes according to another embodiment of the invention
• Figure 4 shows an array of microelectrodes according to another embodiment of the invention
• Figures 5a and 5b show the placement of a diamond based microelectrode array in a housing, according to a preferred embodiment of the invention
• Figure 6 is an electrical schematic diagram showing a preferred circuit layout for a sensor, according to the invention
• Figure 7 shows a sensor based on a microelectrode array according to a preferred embodiment of the invention
• Figure 8 is a schematic representation of a wellbore
• the present invention is embodied in devices preferably based on diamond-based arrays of microelectrodes. Using diamond-based arrays of microelectrodes, redox active species can be detected and measured. Such diamond-based array sensors can advantageously be deployed in the oilfield environment where such redox active species measurement and detection are often critical to activities such as well-drilling, formation evaluation and production processes.
• a non-conductive substrate is provided which is composed of intrinsic diamond, and one or more conductive portions are provided composed preferably of boron-doped diamond.
• the invention preferably makes use of diamond devices manufactured using high precision manufacturing techniques such as described in co-pending patent application filed in the UK Patent Office on 4 August 2003 by applicant Element Six Limited entitled “Diamond Microelectrodes", which is incorporated herein by reference.
• a series of such devices are provided, where a non-conducting (preferably intrinsic) diamond surface containing multiple coplanar areas of conducting diamond.
• the areas of conducting diamond are preferably in electrical communication with each other and are separated on main surface of the non-conducting diamond.
• Diamond-based sensors described herein have a number of advantages over conventional sensors, such as the following. 1. An all-diamond structure is well suited for application in extremely harsh environments such as that of a well-bore. In particular, diamond-based sensors are well suited for operation over an extended range of elevated temperatures and pressures. Thus, providing a relatively long service time which can include multiple usages . 2. The diamond-based sensors described herein provide significantly higher signal-to-noise ratio than conventional macroelectrodes. 3. The total current output is a sum of individual microelectrodes (i.e. there is no significant overlapping in the diffusion spheres of neighbouring electrodes) , hence considerably larger current scale is provided that generally falls in the range of ready measurement without the need for complex electronic circuits . 4.
• the diamond-based sensors described herein provide significantly higher signal-to- (capacitively coupled) interference ratio than single microelectrode. 5.
• the diamond-based sensors described herein are relatively free from current leakage between individual conducting domains, which is important for epoxy-based microelectrode and its arrays .
• the sensors described allow rapid attainment of the steady state in mass transport, and allow relatively fast potential scan ( ⁇ 100V/s) without distortion in the i-V characteristics.
• the sensors described are useful in highly resistive and/or viscous media such as crude oil.
• the use of diamond materials for electrodes advantageously provides a wide range of operation potentials for monitoring redox reactions.
• Figures la and lb show a diamond-based microelectrode array according to a preferred embodiment of the invention.
• Figure la is a cross-section of microelectrode array 100 along the line A-A' as shown in Figure lb which is a plan view of microelectrode array 100.
• Diamond layer 121 is non-conducting preferably intrinsic diamond and may be single crystal or polycrystalline in structure. Diamond layer 121 will typically be synthetic although natural diamond could also be used. Synthetic diamonds used in the present invention include high-pressure high-temperature (HPHT) diamond, as well as chemical vapour deposition (CVD) diamond.
• HPHT high-pressure high-temperature
• CVD chemical vapour deposition
• the upper surface 123 of diamond layer 121 will generally be smooth and preferably polished to a surface roughness of less than lOOnm Ra.
• the upper surface area of conducting microelectrodes 112, 114 and 116 are coplanar with surface 123 of diamond layer 121.
• Microelectrodes 112, 114 and .116 are preferably boron (or S, P) doped diamond. Diamond microelectrodes 112, 114 and 116 are electrically connected to a lower portion 110 which is preferably nonconducting intrinsic diamond. The doping of microelectrodes 112, 114 and 116 is performed either during synthesis or subsequently via implantation. According to alternative embodiments of the invention, lower portion 110 is made of a non-diamond material such as graphite, which may be grown or implanted or metal which may be deposited using any known techniques (vapour deposition, sputter deposition, laser ablation, a diamond growth substrate that has not been removed, electroplating or implantation) .
• the vertical length of the microelectrodes 112, 114 and 116 i.e. the distance from the exposed upper surface to the top of the lower portion 110,- is preferably greater than 1 mm. It has been found that providing a length of 1mm or greater improves the dynamic range of electric potential values for the sensing device.
• Figure lb shows a plan view of a hexagonal coplanar arrangement of microelectrodes - note that the microelectrodes, including microelectrodes 112, 114 and 116 and the other microelectrodes are arranged in a hexagonal geometrical pattern in layer 121.
• the hexagonal arrangement shown is preferable because it allows for a relatively large spacing between microelectrodes for a given number of microelectrodes (in this case, seven) and a given surface area. In general it is preferable to maintain a certain spacing between microelectrodes so as to increase the volume from which diffusion will allow interaction with an electrode (the "diffusion sphere") . In general, it has been found that the distance between neighbouring microelectrodes should at least five and preferably about ten times larger than the diameter of the individual electrode surfaces.
• the general rule would be to space the microelectrodes apart more than five and preferably ten times the diameter of a circle having the same surface area the non-circular microelectrodes.
• the general design rule of ten times the diameter is followed, in many applications the diffusion spheres of the microelectrode areas do not overlap, but the number of microelectrodes is high enough for a given surface area such that the signal to noise ratio is significantly enhanced over conventional arrangements .
• the term microelectrode refers to electrodes that have a relatively small surface area.
• each circular microelectrode has a diameter of 100 microns or less. Even better signal to noise ratios can be obtained with 50 micron diameters and even smaller diameters, such as 25 microns.
• the exposed surface area of the non-circular microelectrodes should be less than 8000 sq. microns, and preferably less then 2000 sq. microns, and even more preferably less than 500 sq. microns. In general the lower limit of the electrode surface size will be largely due to limitations of the process technologies used. Although seven microelectrodes are shown in
• Figure lb other numbers can be used.
• two or more microelectrodes will provide greater sensitivities in particular applications. With greater numbers of microelectrodes, the signal strength will be greater, thereby placing less demand on the amplification circuitry required.
• the design of multiple microelectrodes is more robust and well suited for applications such as the downhole environment. It has been found that providing from 7 to 19 microelectrodes allows for a reasonable signal strength and redundancy for many oilfield-related applications.
• Figure lc shows a microelectrode array according to another embodiment of the invention.
• Array 100' is shown with a hexagonal pattern of 73 microelectrodes.
• any number of microelectrodes can be used, and greater numbers of microelectrode areas should be provided when for applications requiring detection of very low concentrations of analytes.
• array refers to a plurality of elements not necessarily arranged in a regular pattern.
• a non-regular distribution of microelectrode area can be provided, in some cases the spatial distribution of the microelectrode array can be random.
• Figure Id shows an example of a microelectrode array 100" having a square pattern, according to another embodiment of the invention.
• Figure 2 show an arrangement of microelectrodes in a diamond layer according to another embodiment of the invention.
• diamond layer 121 is preferably non-conducting intrinsic diamond and may be single crystal or polycrystalline in structure.
• Diamond layer 121 will typically be synthetic although natural diamond could also be used.
• Synthetic diamonds used in the present invention include high-pressure high-temperature (HPHT) diamond, as well as chemical vapour deposition (CVD) diamond.
• HPHT high-pressure high-temperature
• CVD chemical vapour deposition
• the upper surface 123 of diamond layer 121 will generally be smooth and preferably polished to a surface roughness of less than lOOnm Ra.
• Microelectrodes 150, 152 and 154 are not electrically connected to a lower .layer as in Figures la and lb, rather they are individually addressable. Thus the microelectrodes may be used to sense different chemical properties or chemical species if they are each coated with different functional coatings as described herein below. For example, through different modifications as described below, microelectrodes 150 and 152 could be made to probe different target species. Electrodes 150, 152 and 154 are preferably made from boron doped diamond and arranged in a hexagonal layout, as described above, but could also be made by other doping techniques, or using other materials, or other geometrical arrangements as also described herein.
• FIG. 3 shows an array of microelectrodes according to another embodiment of the invention. In this embodiment the surfaces of the microelectrodes, for example microelectrode 160 are recessed below the surface 123 of diamond layer 121. Microelectrodes as shown in Figure 3 have a reduced or more restricted diffusion sphere volume which may be desirable in some applications.
• Figure 4 shows an array of microelectrodes according to another embodiment of the invention. In this embodiment the microelectrodes 170, 172 and 174 protrude above the surface 123 of diamond layer 121. In addition, the shape of the exposed microelectrodes is rounded to a spherical shape.
• Microelectrodes as shown in Figure 4 have the advantage of enhancing the size of the diffusion sphere volume for each microelectrode which may be desirable in some applications .
• the materials and arrangements of the microelectrodes and the underlying layer 110 are preferably as described above with respect to Figures la and lb.
• the surface of the microelectrodes can be bare, i.e. unmodified, wherein the boron-doped diamond alone is the reacting surface. This may be suitable for example to sensing the presence of hydrogen sulphide.
• For an example of sensing hydrogen sulphide with a bare reaction surface see co-pending PCT patent application number PCT/GB2003/002345, incorporated herein by reference.
• the surfaces of the microelectrodes are preferably modified or functionalised so as to be particularly sensitive to certain species or chemical properties.
• the modification can be achieved either by monolayer coverage or by polymer layers up to micrometer thickness .
• Surface modifications of the boron-doped diamond microelectrodes . can be performed by one of several different means.
• Metal oxide nanoparticles can be adsorbed onto the boron-doped diamond microelectrodes, as for example described by McKenzie et al . (Electrochemistry Communications, volume 4, page 820, 2002). Further derivatisation of the metal oxide particles can be achieved, such as complexation with carboxylate- or thiol-containing ligands .
• metals may be deposited onto the surface of the microelectrodes using one of a variety of techniques, such as low-temperature plasma or direct metal evaporation/condensation.
• Pitter et al . Applied Physics Letters, volume 69, page 4035, 1996) used a direct evaporation/condensation technique to deposit silver metal onto boron-doped diamond; the silver metal deposited at ambient temperature formed small islands on the electrode surface.
• the metal or metal oxide deposit on. the surface of boron-doped diamond microelectrodes can be used to further modify the electrode surface.
• alkyl thiols such as 1- octanethiol
• hydrophilic surfaces can be generated using thiol- terminated carboxylic acids or amines, such as mercaptoacetic acid or a 4-mercaptopyridine; these derivations enable the surface to be either negatively or positively charged.
• the surface of the boron-doped diamond microelectrodes can be directly functionalised by introducing oxygen to terminate the surface carbon atoms .
• Nagao et al . Japanese Patent J. Applied Physics. Part 2.
• the oxygen terminals of the boron-doped diamond can be used to graft a variety of functional groups onto the electrode surface.
• the surface can be made hydrophobic by reaction with chlorodimethyloctylsilane, which generates a C-O-Si- linkage to graft the hydrophobe onto the electrode surface.
• other functionalisations are possible to graft ionophores and other redox centres onto the surface.
• the diamond working electrode surfaces are modified using N, N ' -dimethylphenylenediamine (DMPD) , or a structural analogue, together with a conducting sphere of micrometer scale (carbon or boron carbide), or nanometer scale (carbon nanotubes, or metal nanoparticles) .
• DMPD N, N ' -dimethylphenylenediamine
• these species can be spikes together with a thin layer of microporous epoxy with certain ratio, thus leads to a all-solid state, functionalised electrode surface that is sensitive to the concentration of hydrogen sulfide. See, co-pending GB Patent Application number 0217249.2, filed 25 July 2002, incorporated herein by reference.
• the diamond microelectrodes are used to measure pH by modifying the working electrode surfaces through the reduction of aryl diazonium salts.
• aryl diazonium salts For example, see Kuo et al . (Electrochem . & Solid-State Lett . , volume 2, page 288, 1999) .
• Derivatives of anthraquinone can be grafted onto the boron-doped diamond electrode to yield a pH electrode, as for example achieved by Ojani et al . on carbon paste electrodes ⁇ Iran . J. Chem . & Chem . Eng. , volume 20, page 75, 2001) using the physical mixing of anthraquinone derivatives with carbon paste.
• the diamond based sensor is used to sense non-chemical fluid properties such as resistivity.
• the diamond microelectrodes can be used to measure the redox behaviour and conductivity of highly resistive liquids, such as oils and lubricants.
• Kauffman US
• FIGS 5a and 5b show the placement of a diamond based microelectrode array in a housing, according to a preferred embodiment of the invention.
• Microelectrode array 100 is preferably as described in Figures la and lb, but may also be as elsewhere described herein including in association with Figures lc, Id and 2-4.
• microelectrode array 100 is assembled into an electrochemical device 180, in which the diamond based microelectrodes are used as the working electrode.
• Device 180 also preferably comprises a counter electrode 204 (preferably made of platinum) and a reference electrode 206 (preferably made of Ag ⁇ AgCl or Ag ⁇ AgI or a short piece of platinum as pseudo-reference) .
• a counter electrode 204 preferably made of platinum
• a reference electrode 206 preferably made of Ag ⁇ AgCl or Ag ⁇ AgI or a short piece of platinum as pseudo-reference
• the microelectrode array 100 is constructed on top of a substrate 202 which is preferably made of polyetheretherketone (PEEK) material.
• FIG 5b shows a perspective view of electrochemical device 180.
• electrodes 210, 212 and 214 are electrically connected to, respectively, the counter electrode 204, reference electrode 206 and working electrode, which consists of microelectrodes 100 as shown in Figure 5a.
• Figure 6 is an electrical schematic diagram showing' a preferred circuit layout for an electrochemical sensor, according to the invention. The electrical connections 210, 212 and 214 to, respectively, the counter electrode, reference electrode and working electrode are shown.
• the output signal can be used to indicate the particular species and/or chemical properties according to the type of microelectrode array being used.
• the electronics shown in Figure 6 can be obtained commercially from vendors such as Alphasense Limited (www.alphasense.com) .
• Figure 7 shows an electrochemical sensor based on a microelectrode array according to a preferred embodiment of the invention.
• the sensor 300 comprises a generally cylindrical housing 340, which is preferably made from PEEK and which comprises a main housing member 342 having an upper portion 344, a reduced diameter lower portion 346, and a stepped diameter cylindrical bore 348 extending coaxially through it from top to bottom.
• the bore 348 has a large diameter upper portion wholly within the upper portion 344 of the main housing member 342, an intermediate diameter portion also wholly within the upper portion of the main housing member, and a reduced diameter portion largely within the lower portion 346 of the main housing member.
• a flowpath 356 for the fluid to be sensed extends diametrically through the upper portion 344 of the main housing member 342, intersecting the upper portion 350 of the bore 348.
• Disposed in the intermediate diameter portion of the bore 348, and resting on the shoulder defined between the reduced diameter portion and the intermediate diameter portion, is a cylindrical electrochemical device 180 as described more fully above.
• An O-ring made of VITONTM is disposed in a groove extending coaxially round the body of device 180 to seal the device within the intermediate diameter portion of the bore 348.
• a cylindrical membrane retainer assembly 376 Disposed in the large diameter upper portion of the bore 348, and resting on the shoulder defined between the intermediate diameter portion and the large diameter portion is a cylindrical membrane retainer assembly 376, which comprises a cup-shaped housing member, a cylindrical housing member which screws part of the way into the cup-shaped housing member, and a gas permeable membrane 382 preferably in the form of a circular plate made of zeolite or other suitable ceramic material coaxially located in the cup-shaped housing member, in the space between the bottom of the inside of the cup shape of the housing member and the bottom of the cylindrical housing member.
• the cylindrical housing member has a diametrically extending flow path therethrough being aligned with the flow path 356 in the upper part 344 of the main housing member 342.
• sensor 300 is adapted to sense hydrogen sulphide.
• the generally cylindrical space 394 beneath the underside of the membrane 382 and the top of the device 180 constitutes a reaction chamber, and is filled with a reaction solution containing a precursor or catalyst, for example, dimethylphenylenediamine (DMPD) .
• DMPD dimethylphenylenediamine
• membrane 382 is not provided. In many applications it is better not to use a membrane, since mass transfer is faster and direct contact between the microelctrodes and the fluid allow for greater accuracy in measurement of concentration or chemical property.
• FIG. 8 is a schematic representation of a wellbore tool which is positioned in a wellbore and which is equipped with an electrochemical sensor in accordance with the present invention.
• the wellbore tool shown in Figure 8 is indicated at 410, and is based on
• the tool 410 comprises an elongated substantially cylindrical body 412, which is suspended on a wireline 414 in the wellbore, indicated at 416, adjacent an earth formation 418 believed to contain recoverable hydrocarbons, and which is provided with a radially projecting sampling probe 420.
• the sampling probe 420 is placed into firm contact with the formation 418 by hydraulically operated rams 422 projecting radially from the body 412 on the opposite side from the sampling probe, and is connected internally of the body to a sample chamber 424 by a conduit 426.
• a pump 428 within the body 412 of the tool 410 can be used to draw a sample of the hydrocarbons into the sample chamber 424 via the conduit 426.
• the pump is controlled from the surface at the top of the wellbore via the wireline 414 and control circuitry (not shown) within the body 412.
• this control circuitry also controls valves (not shown) for selectively routing the sampled hydrocarbons either to the sample chamber 424 or to a dump outlet (not shown) , but these have been omitted for the sake of simplicity.
• the conduit 426 additionally communicates with an electrochemical sensor 300 also provided within the body 412 of the tool 410, so that the hydrocarbons flow over a face of the sensor on their way through the conduit.
• the sampling probe is located close to the electrochemical sensor 300, at a distance comprised between 8 and 30 cm from said electrochemical sensor, advantageously approximately equal to 15 cm.
• the sensor 300 produces an output current, which is dependent on the amount of species or chemical property sensor 300 is adapted to detect in the hydrocarbons flowing through the conduit 426.
• a digital current measuring circuit 432 (as , described in connection with Figure 6) in the body 412 of the tool 410, and the measurement is transmitted to the surface via the wireline 414.
• Figure 8 depicts an open hole sampling tool, it will be recognized that the present invention is also applicable for use with downhole sampling tools for cased hole as well.
• the sensor 300 is integrated for use with the Cased Hole Dynamics Tester (CHDT) tool from Schlumberger . See, e.g. the CHDT product brochure: http: //www.hub. sib.
• CHDT Cased Hole Dynamics Tester
• Figure 9 shows a drilling system using a diamond-based sensor, according to a preferred embodiment of the invention.
• Drill string 558 is shown within borehole 546.
• Borehole 546 is located in the earth 540 having a surface 542. Borehole 546 is being cut by the action of drill bit 554.
• Drill bit 554 is. disposed at the far end of the bottom hole assembly 556 that is attached to and forms the lower portion of drill string 558.
• Bottom hole assembly 556 contains a number of devices including various subassemblies .
• measurement-while-drilling (MWD) subassemblies are included in sensor subassembly 562.
• a subassembly 562 is provided to make measurements using a diamond based sensor as herein described.
• the signals from the sensor subassembly 562 are preferably communicated to pulser assembly 564.
• Pulser assembly 564 converts the information from subassembly 562 and other subassemblies into pressure pulses in the drilling fluid.
• the pressure pulses are generated in a particular pattern which represents the data from the subassemblies.
• the pressure pulses travel upwards though the drilling fluid in the central opening in the drill string and towards the surface system.
• the drilling rig 512 includes a derrick 568 and hoisting system, a rotating system, and a mud circulation system.
• the hoisting system which suspends the drill string 558 includes traveling block and hook 572 and swivel 574.
• the rotating system includes kelly 576, rotary table 588, and engines (not shown) .
• the rotating system imparts a rotational force on the drill string 558 as is well known in the art.
• a system with a Kelly and rotary table is shown in Figure 9, those of skill in the art will recognize that the present invention is also applicable to top drive drilling • arrangements.
• the drilling system is shown in Figure 9 as being on land, those of skill in the art will recognize that the present invention is equally applicable to marine environments.
• the mud circulation system pumps drilling fluid down the central opening in the drill string.
• the drilling fluid is often called mud, and it is typically a mixture of water or diesel fuel, special clays, and other chemicals.
• the drilling mud is stored in mud pit 578.
• the drilling mud is drawn in to mud pumps (not shown) , which pumps the mud though stand pipe 586 and into the kelly 576 through swivel 574 which contains a rotating seal.
• mud pumps not shown
• gas is introduced into drilling mud using an injection system (not shown) .
• the mud passes through drill string 558 and through drill bit 554.
• a diamond-based sensor 520 is mounted on sensor subassembly 562. If fluid to be monitored is the wellbore fluid passing upwards to the surface, the sensor 520 is mounted on or near the outer surface of the subassembly so as to be exposed to wellbore fluids passing upwards toward the surface.
• sensor 520 is adapted to sense hydrogen sulfide as herein described.
• Blowout preventer 599 comprises a pressure control device and a rotary seal .
• the mud return line feeds the mud into separator (not shown) which separates the mud from the cuttings. From the separator, the mud is returned to mud pit 578 for storage and re-use. Mud pulses traveling up the drillstring are detected by pressure sensor 592.
• Pressure sensor 592 comprises a transducer that converts the mud pressure into electronic signals.
• the pressure sensor 592 is connected to surface processor 596 that converts the signal from the pressure signal into digital form, stores and demodulates the digital signal into useable MWD data.
• Figure 10 shows a diamond based sensor incorporated into a production logging tool to monitor fluid in a horizontal section of a well.
• Production logging tool 612 is shown positioned on a wireline within horizontal section 610 of a well in formation 650.
• the horizontal section 610 may be either cased or open hole.
• the production logging tool includes a number of separate sensors for taking independent measurements such as total flow rate, phase velocity, flow imaging, water flow, etc.
• Preferably a number of diamond based sensors are mounted near centralizer 620.
• diamond based sensors 618a and 618b are mounted on separate members just inside different arms of centralizer 620. Mounting the sensors in this advantageously allows for different parts of the flow to be monitored, such as would be useful when the flow in the well is stratified.
• a diamond bases sensor 616 is mounted on' a subassembly 614 on the main body of the logging tool 612. According to one preferred embodiment the diamond bases sensors are adapted to sense hydrogen sulfide as is described herein.
• Figure 11 shows a diamond based sensor used to monitor fluid flowing in a conduit, according to embodiments of the invention. The fluid to be monitored flows through conduit 710 in the direction indicated by arrow 712.
• Sensor housing 722 is provided to house sensor body 724 which includes a diamond-based micro electrode structure 726 as described herein. • The electrical signals from the microelectrode structure is interpreted by processed 720 as described herein.
• conduit 710 carries wellbore fluid and is placed either downhole or on the surface of an oil well. The sensor is used to sense fluid properties such as resistivity or pH, or particular chemical species such as hydrogen sulphide as herein described.
• conduit 710 is part of a chemical processing facility and sensor 724 is adapted to sense fluid properties or chemical species relevant to a chemical processing application.
• the diamond based sensor can be used to sense various chemical species, chemical properties such as pH, and other characteristics of the fluid in conduit 710 such as resistivity.
• the sensor 724 is used for environmental monitoring.
• conduit 710 is used for C02 sequestration using wellbores and sensor 724 is used for monitoring pH as is described herein.
• sensor 724 is used to monitor hydrogen sulphide when monitoring volcanic activity.

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